Abstract

Strong thermo- and photoacoustic responses have been detected for aligned arrays
of multi-walled carbon nanotube (MWNT) forests and solid drawn MWNT sheets.
When heated using alternating current or a near-IR laser modulated in 100-20
000 Hz range, the nanotube assemblies generated loud, audible sound, with higher
sound pressure being detected from the MWNT sheets. An evaluation of nonlinear
distortions of the thermoacoustic signal revealed a highly peculiar behavior
of the third and fourth harmonics produced from forests grown on silicon wafers.
The peculiarities were especially pronounced for short forests and can be associated
with the heat transfer from the MWNT layer to the substrate. The dependencies
can be used for evaluating properties of thermal interfaces, in particular,
those based on carbon nanotubes. The investigated nanotube sheets can be attached
to any surface (e.g., wall of an apartment) making the surface acoustically
active. They can also be attached to windows of buildings and cars and can be
used for active noise cancellation in noisy environments. Because of good coupling
with the surrounding air, MWNT sheets and forests can be used for dissipation
of heat produced by various electronic devices. They can be easily patterned
in such way, that upon scanning with a laser beam, the pattern generates a characteristic
sound and can therefore be employed used for acoustically readable barcodes.

Introduction

Remarkable properties of carbon nanotube assemblies (forests, sheets, yarns)
are expected to lead to a variety of applications. It has recently been reported1
that freestanding multi-walled carbon nanotube (MWNT) sheets generate sound
when heated with alternating current (ac).

Thermoacoustic (TA or current driven) properties are related to the widely
studied photoacoustic (PA or laser radiation driven) behavior. In order to check
if the outstanding performance of this unusual material can be achieved with
the laser excitation, we carried out the comparative TA-PA measurements of single
(S1) and multilayer MWNT sheets (S2-S10) [2]. We also discovered that quite
strong TA and PA signals can be generated by vertically aligned arrays of MWNTs
(MWNT forest) grown on silicon wafers. In spite of their topological differences
from the sheets, heating the forests with alternating current or a laser beam
modulated in the audio frequency range produced an acoustic signal that can
be heard by ear or detected with a microphone. We also performed measurements
of nonlinear distortions of the TA signal and discovered highly peculiar behavior
of the third and fourth harmonics produced by forests from ac-dc voltage scans
[2]. The reported data can find applications in devices for sound generation
and manipulation, heating, and cooling.

Results

Aligned arrays of MWNTs (MWNT forest) were grown by the conventional chemical
vapor deposition technique on silicon wafers and had heights of about 32, 138,
and 233 µm (labeled F32, F138, and F233). MWNT sheets were fabricated
by pulling a sidewall of the forest in the sample plane direction, Figure 1A.

Figure1:
MWNT forest in the process of conversion into MWNT sheet (A). Sound
pressure generated by the MWNT sheets (S1-S10) and MWNT forests
(F32-F233) subjected to TA excitation (B). The data were taken
at 5000 Hz, for the first harmonic of the acoustic signal.

The sound pressure generated by MWNT sheets and MWNT forests subjected to TA
excitation is shown in Figure 1B. TA intensities were normalized by the supplied
electric power. The TA efficiency (sound pressure per watt of input power) was
the highest for the single layer MWNT sheet: about 820 mPa/W (92 dB/W). Although
the sound pressure generated by the multilayer sample in our experiments was
roughly proportional to the number of layers, the conversion efficiency for
multilayer samples dropped quite significantly, to about 63% for S10 as compared
with S1. This could be caused by some increase in the sheet density in the multilayer
assembly.

TA sound pressure for the investigated MWNT forests was comparable to that
for the S1 sample. However, the efficiency of sound generation by forests was
found to be lower than the efficiency of the MWNT sheets. The highest value
was recorded from the F138 forest: 153 mPa/W (78 dB/W), which is 19% the efficiency
of S1 sample. This difference can be caused by the substantially higher forest
density compared with the density of MWNT sheets.

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